Wednesday, January 11, 2012

Jellyfish on the Beach

The shoreline is always a fun place to visit; not only is it a picturesque confluence of all sorts of sedimentary and geomorphic processes mingling and interacting with on another, but there's all sorts of wiggy critters and nifty biology to see as well. Over the break, I had the chance to spend a lovely day at the beach on St. George's island, a barrier island in the Gulf of Mexico not too far from the Oyster Capital of the Gulf, Apalachicola Florida. Some recent storms had stirred up the shoreface a bit, resulting in some pretty nifty shell hashes and fragments scattered all over. However, along with the biomineralized detritus cast up by storm waves, there were also a fair number of these guys:

Jellys! Subaerially exposed! Some of these, like the one in the picture above, were exposed in the littoral zone, but a few of comparable size had been chucked up pretty far onto the beach, out of range of fairweather wave activity. Always fun to see cnidarians, especially when the chance of getting stung in minimal. Anyway, seeing all these jellys, deposited on well-sorted, upper fine- to medium- grained sand got me thinking about a taphonomy...after all, Jellys aren't called Jellys out of some wry sense of irony. These soft, squishy, blobby little guys, chucked up into high-energy settings like the foreshore, don't have much of a chance at preservation, especially with a bunch of other critters rambling about over the beach. However, such was not always the case!

A similar occurrence of cnidarians and high-energy deposits has been documented from the Cambrian, with some very nice examples coming out of the midcontinent region in particular. Hagadorn et al. (2002) documented a pretty awesome occurrence from the "middle" Cambrian Mt. Simon/Wonewoc sandstones. Below is their Figure 3, from page 149.

Pretty cool! It's a little more densely packed with the tragically stranded cnidarians, and the sedimentary structures are different, but all in all, it's pretty much a Cambrian example of my day at the beach!

Hagadorn et al. (2002) noted that, as discussed above, the taphonomy of jelly preservation requires some sort of explanation. First off, these deposits don't show any evidence for rapid burial, one of the more commonly evoked explanations for exceptional fossil preservation. Rather, these jellyfish show themselves, on the basis of both sedimentary structures and the overall stratigraphic succession, to have been deposited in a fairly active shoreface/foreshore paleoenvironment. Additionally, there's no evidence in these rocks for microbial structures or features, meaning that we can't evoke a "gooey substrate" explanation, either. So, how did these guys get into the rock record!?

An important hint, recognized by Hagadorn et al. (2002), can be found in the explanation of why washed up jelly's DON'T get preserved today...namely, lots of scavenging terrestrial critters and especially lots of deep digging bioturbators. One of the ways the weird world of the Cambrian differs from ours today is the fact that the organisms that do those things hadn't evolved yet, meaning that, for a brief window of time, these sort of "stranding" deposits of organisms are actually pretty darn likely to enter the stratigraphic record.

Just goes to show you that, in addition to our uniformitarian world view, we also have to keep in mind the arrow of secular variation, and how dynamic and complex the interaction is between critters and their environments.